This accessory block of metal, plastic or other suitable material, will convert a mortise lock of the springbolt rimlatch type with a bolt having a flat or domed bevelled face which is self latching when the door is closed, into a deadbolt lock which must be opened and closed with a key. The deadbolt lock is generally regarded as the more secure door lock. The mortise springlatch lock is not secure and can easily be opened from the outside by shimming (loiding) with a credit card or other material such as semi-rigid cellophane. Anti-theft programs recommend that the springlatch door lock be replaced with a deadbolt door lock. This is an expensive and time consuming exchange. It would be desirable to have some way to convert the existing springlatch door lock into a deadbolt type. This invention does that by the use of a five or six sided prismatic block of suitable dimensions and adhesives which have only recently become available. The principle involved is one of removing the bevelled face, for without a bevelled face the shimming technique will not work. The bevelled face is mated to a prism or wedge-type block which can be solid or hollowed and when suitably attached to the springlatch the bevel disappears. Using this technique the springlatch bolt is changed into a deadbolt with a minimum of expense and effort.